ValidTicTacToeState.java

package array;

/**
 * Created by gouthamvidyapradhan on 29/03/2019
 * A Tic-Tac-Toe board is given as a string array board. Return True if and only if it is possible to reach this
 * board position during the course of a valid tic-tac-toe game.
 *
 * The board is a 3 x 3 array, and consists of characters " ", "X", and "O".  The " " character represents an empty
 * square.
 *
 * Here are the rules of Tic-Tac-Toe:
 *
 * Players take turns placing characters into empty squares (" ").
 * The first player always places "X" characters, while the second player always places "O" characters.
 * "X" and "O" characters are always placed into empty squares, never filled ones.
 * The game ends when there are 3 of the same (non-empty) character filling any row, column, or diagonal.
 * The game also ends if all squares are non-empty.
 * No more moves can be played if the game is over.
 * Example 1:
 * Input: board = ["O  ", "   ", "   "]
 * Output: false
 * Explanation: The first player always plays "X".
 *
 * Example 2:
 * Input: board = ["XOX", " X ", "   "]
 * Output: false
 * Explanation: Players take turns making moves.
 *
 * Example 3:
 * Input: board = ["XXX", "   ", "OOO"]
 * Output: false
 *
 * Example 4:
 * Input: board = ["XOX", "O O", "XOX"]
 * Output: true
 * Note:
 *
 * board is a length-3 array of strings, where each string board[i] has length 3.
 * Each board[i][j] is a character in the set {" ", "X", "O"}.
 *
 * Solution: Do a brute-force check for each row, column and diagonals and keep track of count of 'X' and 'O'
 */
public class ValidTicTacToeState {
    /**
     * Main method
     * @param args
     */
    public static void main(String[] args) {
        String[] board = {"XXX", "XOO", "OO "};
        System.out.println(new ValidTicTacToeState().validTicTacToe(board));
    }

    public boolean validTicTacToe(String[] board) {
        boolean xWon = hasWon(board, 'X');
        boolean oWon = hasWon(board, 'O');
        int xcount = 0, ocount = 0;
        for(int i = 0; i < 3; i ++){
            for(int j = 0; j < 3; j ++){
                if(board[i].charAt(j) == 'X'){
                    xcount++;
                }else if(board[i].charAt(j) == 'O'){
                    ocount++;
                }
            }
        }
        if(xWon && oWon) return false;
        if(xWon){
            return ((xcount - 1 == ocount));
        } else if(oWon){
            return ((xcount == ocount));
        } else {
            return (xcount == ocount || xcount - 1 == ocount);
        }
    }

    private boolean hasWon(String[] board, char c){
        boolean diagnol = ((board[0].charAt(0) == c && board[1].charAt(1) == c && board[2].charAt(2) == c) ||
        (board[0].charAt(2) == c && board[1].charAt(1) == c && board[2].charAt(0) == c));
        if(diagnol) return true;
        for(int i = 0; i < 3; i ++){
            if(board[i].charAt(0) == c && board[i].charAt(1) == c && board[i].charAt(2) == c) return true;
        }
        for(int i = 0; i < 3; i ++){
            if(board[0].charAt(i) == c && board[1].charAt(i) == c && board[2].charAt(i) == c) return true;
        }
        return false;
    }
}